This provides a gem that provides a small rpc interface to stagers.
Although this is a bit of yak shaving, it does clean up the interaction
between the CC and Stagers and paves the way for adding support for allowing
staging plugins to perform actions on behalf of users.
Test plan:
- All existing unit tests pass
- New stager_ipc unit tests pass
- BVTs pass
Change-Id: I18d059cbc044fd628680dbb1bfd8428b4ced0c16